The present invention relates to a light-sensitive copying material and to a process for the manufacture of such material, which includes the general steps of coating a support with a solution of the components of the light-sensitive layer which are dissolved in a solvent, preferably in an organic solvent, and drying the layer.
Usually, light-sensitive copying materials of various descriptions are manufactured in such a way that supports consisting of metals, plastic films, paper, fabrics and the like are coated as evenly as possible with solutions of the layer components, and these solutions are then dried under conditions which ensure the formation of layers which are as uniform as possible.
The layer components used generally comprise light-sensitive or photoconductive compounds or combinations of such compounds, binders, dyes, pigments, plasticizers, fillers, polymerizable compounds, stabilizers and the like. The components are preferably dissolved in organic solvents and, occasionally, also in water or mixtures of water and organic solvents.
In industrial scale coating and in continuous manufacturing processes, mixtures of several solvents which have different evaporation numbers are practically always used, in order to obtain a substantially continuous transition from the liquid film phase to the dry layer and to prevent, if possible, any disintegration during drying. In most cases, this is achieved by a combination of solvents having a good dissolving power with non-solvents; the vapor pressures and evaporation numbers of the components are chosen in such a way that the non-solvents are the first to evaporate in the drying process. Thus, layers of an adequate coating quality are produced, but unless they are colorless, these layers still exhibit a certain irregularity which is referred to as "cloudiness".
The term "cloudiness" denotes the presence of contiguous layer areas of lighter and darker shades which have diameters ranging from about 1 to 10 mm. Scanning electron micrographs (SEM) of sections through copying layers have revealed that variations in color density can be attributed to differences in the layer thickness.
It is presumed that, until the gel phase is reached in the drying process, the still liquid layer is subject to whirling effects or agitations of substance, which are caused by changes in the surface tension of the layer in the course of drying. Evaporation of the solvent gives rise to two factors which mutually intensify each other and raise the surface tension of the drying layer at its interface to the ambient air. On the one hand, the temperature drop leads to a rise in surface tension of the order of 0.1 nM/n.degree. C.; on the other hand, surface tension is additionally augmented by the increase in concentration of the film-forming substances contained in the solution.
Emanating from numerous small swelling points in the underlying layer of less concentrated solution, material of higher temperature and lower surface tension, forces its way to the surface. As a result, corresponding centers are formed on the surface and from these centers, the liquid rising from below spreads, until the fronts of the spreading waves of adjacent centers meet. Due to a deviation of the liquid agitation into the interior of the layer, turbulences are produced.
This phenomenon is, to a particularly high degree, encountered in the manufacture of light-sensitive printing plates, especially of planographic printing plates, where relatively highly diluted solutions of low viscosity are frequently used. Layers which are cloudy and have non-uniform thicknesses and colorations are often obtained, and such irregularities in layer-thickness have a negative effect on the printing result, particularly in plates for screenless offset printing. Here, the surface structure of the plate is reproduced on the impression in gray tone steps, i.e., uniform gray areas are not printed.
From Research Disclosure (1976) 145, page 15, No. 14522, it is known to add a combination of a non-ionic surfactant, i.e., isononyl phenoxy decaglycidol, and an anionic surfactant, i.e., an ammonium salt of sulfated nonyl phenoxy poly(ethyleneoxy)ethanol, to lithographic layers to improve the coatability of the coating solution and to ensure good inking properties.
According to this description, one surfactant alone is not sufficient to yield the desired properties; the non-ionic surfactant, if used alone, causes a deterioration of ink receptivity and scumming of the background.
German Offenlegungsschrift No. 29 14 558 discloses a coating process in which the uniformity of the light-sensitive layers on planographic printing plates is improved by adding to the coating solution a polymer which has a molecular weight of at least 10,000 and which contains at least one fluoroalkyl group. As stated in the specification, the addition of these polymeric compounds results in a more uniform layer-thickness. Although it is mentioned in the description that a number of solvents can be used, individually or in mixtures, for the preparation of the coating solutions, solvent mixtures exclusively are employed in the examples, and in each case, the mixture contains one low-boiling and one high-boiling solvent.
The use of high-molecular compounds with fluorinated side chains has, in addition, the disadvantage that these polymers are no longer soluble in all customary solvents so that the choice of a solvent is limited.